1. Field of the Invention
This invention relates in general to subsea wellhead equipment, and in particular to a subsea wellhead system that utilizes a tieback to a production platform, and has the ability to remove the inner casing string without removing the intermediate and outer tieback conduits.
2. Description of the Prior Art
In a typical tieback system, a floating drilling rig will initially drill a few wells. These wells will be completed subsea, but initially will not have a production Christmas tree. Then, a platform will be positioned over the wells. Each subsea wellhead housing will be tied back to a production Christmas tree at the platform. The platform has production facilities and has a drilling rig for drilling subsequent wells.
Before tying back, the floating drilling rig will have installed two casing hangers in the subsea wellhead housing, each connected to a string of casing. The larger diameter casing, referred to herein as the intermediate casing, will be connected to the lower or intermediate string casing hanger. The intermediate casing hanger lands on a shoulder in the wellhead housing. A seal seals between the intermediate casing hanger and the bore of the wellhead housing.
The inner string casing hanger, which supports the inner string of casing, typically lands on the intermediate casing hanger. A seal seals the exterior of the inner string casing hanger to the bore of the wellhead housing at a point above the intermediate string casing hanger.
During the tieback procedure, an outer tieback conduit will be connected between the wellhead housing and the platform at the surface. One type connects the tieback conduit to exterior grooves on the wellhead housing, while another type connects to interior grooves. The interior connector type is less expensive than the exterior connector type, and is normally preferred. An intermediate string of casing will extend from the surface equipment on the platform into the bore of the intermediate string casing hanger. Subsequently, an inner string of casing will be lowered from surface equipment at the platform into the bore of the inner casing hanger and secured. Tubing will then be placed within the inner string for producing the well. The Christmas tree will be at the platform.
If the well production declines after a few years, the operator may wish to drill another well by sidetracking. In this procedure, the operator will cut the inner string above the level of cement and pull the inner string from the well. He will go back in with drilling equipment to sidetrack and drill a deviated well from the initial well at a point above the upper end of the remaining casing in the well. In this manner, the operator will be able to utilize the existing subsea wellhead housing, intermediate casing string, and intermediate casing hanger.
One problem in performing this task, however, is in retrieving the inner string of casing. If the tieback is of a type that connects the outer tieback conduit to the bore of the wellhead housing, then the inner string casing hanger cannot be pulled upward from the wellhead housing without first removing the outer tieback conduit. The intermediate string tieback conduit would also have to be removed. If the outer tieback conduit is of the type which connects to exterior grooves on the wellhead housing, all strings of tieback conduit except the outer string must be removed. Whether the outer tieback connector is interior or exterior, either procedure is time consuming, requires killing of the well and removal of the surface wellhead equipment.